1. Technical Field
The present invention relates to a pneumatic tire in which both steering stability and road noise-reducing ability are enhanced.
2. Background Technique
Conventionally, in order to enhance noise-reducing ability, especially road noise-reducing ability of a tire, a material having low rigidity is used for a carcass cord constituting a carcass, or the strike number per unit width of the cord is reduced to reduce the rigidity of the tier, thereby restraining noise and vibration from generating. This is because that if the rigidity of the tire is low, input from a road surface can be moderated, and vibration of a tread portion itself transmitted to the road surface can be reduced.
However, in this conventional method, although the road noise-reducing ability is enhanced by reducing the rigidity of the tire, there is a problem that the steering stability is deteriorated. The road noise-reducing ability and the steering stability are in an antinomic relation, and it is extremely difficult to enhance both of them to high level.
In view of the above circumstances, the present inventors researched and as a result, the inventor found that there exists a range where both the steering stability and the road noise-reducing ability can be enhanced by                1) forming a carcass of two carcass plies,        2) differentiating complex elastic moduli of topping rubbers used for the carcass plies from each other, and        3) limiting relative ratio of the complex elastic moduli.        
Each of the following patent documents 1 to 4 proposes a technique for enhancing the road noise-reducing ability while keeping the steering stability by differentiating thermal shrinkage ratio, intermediate elongation, nenier number (cord thickness) or strike number of carcass cords of two carcass plies in a tire having the two carcass plies.
[Patent document 1]
Japanese Patent Application Laid-open No. H8-53003
[Patent document 2]
Japanese Patent Application Laid-open No. H8-164707
[Patent document 3]
Japanese Patent Application Laid-open No. H8-72503
[Patent document 4]
Japanese Patent Application Laid-open No. H8-108704
Based on an idea that a carcass is formed of two carcass plies and complex elastic moduli of topping rubbers of the carcass plies are differentiated with a predetermined ratio, it is an object of the present invention to easily provide a pneumatic radial tire capable of enhancing the steering stability and the road noise-reducing ability without deteriorating uniformity.
To achieve the above object, the present invention provides a pneumatic radial tire having a carcass including radially inner and outer carcass plies each having a ply body extending from a tread portion to a bead core of a bead portion through a sidewall portion, wherein
each of the inner and outer carcass plies includes a cord-arrangement body in which carcass cords are aligned to each other, front and back surfaces of the cord-arrangement body are coated with topping rubber, a complex elastic modulus E*1 of the topping rubber of the inner carcass ply is different from a complex elastic modulus E*2 of the topping rubber of the outer carcass ply, and a ratio E*1/E*2 is 0.95 or lower or 1.05 or higher.
It is preferable that the ratio E*1/E*2 is 0.8 to 0.95 or 1.05 to 1.3. It is also preferable that lower one of the complex elastic moduli E*1 and E*2 is in a range of 4.0 to 5.0 Mpa.